A biological membrane (biomembrane) is an enclosing, or separating amphipathic layer, which acts as a barrier within or around a cell. A biomembrane is usually a lipid bilayer composed of a double layer of lipid-class molecules with proteins intertwined.
Biomembranes typically define enclosed spaces or compartments in which cells maintain a chemical or biochemical environment that differs from the outside. One important feature of biomembranes is that they are selectively permeable structures. The size, charge, and other chemical properties of the molecules that make up the biomembrane determine whether other molecules can pass through the biomembrane. Biological membranes also have certain mechanical or elastic properties.
The selective permeability and mechanical or elastic properties of the cell are determined by the type of lipids and proteins that make up the biomembrane. There are three classes of lipids that are commonly found in biomembranes: phospholipids, glycolipids, and cholesterols. The relative composition of each of these lipids in any given biomembrane depends on the type of cell and other biological factors. A cell membrane, for example, can have thousands of different lipids, some of which may be in high concentration and some of which may be in very low concentrations. Moreover, the lipid composition of biomembranes can change in subtle ways in response to cellular, genetic, chemical, or environmental changes.
Biomembranes such as the cell membrane also include a large amount of protein that gives the cell important characteristics. The amount of membrane protein differs between species and according to function, however the typical amount of protein in a cell membrane is 50%. A membrane protein can be any protein molecule that is attached to, or associated with the membrane of a cell or an organelle. Membrane proteins can be classified into different groups based on the strength of their association with the membrane. An important class of membranes that is highly associated with the membrane is integral membrane proteins, which includes transmembrane proteins.
Biomembranes, including those having integral membrane proteins, are notoriously difficult to study. Often, the amphipathic nature of the membrane proteins makes it difficult to study these proteins in solution. Extracting membrane proteins from the membrane often results in agglomeration, denaturing, and/or deactivation of the membrane proteins. Consequently, membrane proteins are often studied while in the membrane. However, studies of whole membranes are difficult because of the numerous lipids and proteins that are present in biomembranes.